The present invention relates to a coil support system for a transformer and, more particularly, to a coil assembly and support system wherein the conductors forming the primary and secondary windings, or coils, of the transformer are concentrically wound around a section, or leg, of the core of the transformer and encapsulated to form a monolithic structure.
In one particular class of transformer, a pair of a respective plurality of transposed hollow strand conductors, which are typically fabricated from copper and through which fluid may be circulated for cooling the conductors, are cylindrically, concentrically wound around and spirally extend along a core section, or leg, of the transformer to form respective radially spaced apart primary and secondary windings of the transformer. The actual designation of the primary and secondary winding is determined by electrical connections to the windings. For the present invention, it is immaterial which winding is actually the primary and which winding is actually the secondary.
A generally odd number of conductors are transposed to form a winding in order to reduce the effect of leakage flux on anyone conductor. The present invention applies to all windings of a transformer regardless if one or a plurality of conductors form the winding and further regardless if the conductors forming the winding are hollow to permit circulation of coolant fluid therethrough, or solid.
After the conductors are wound around the leg of the core, they must be sufficiently supported both radially and axially to resist and withstand forces due to a fault (such as a sudden short circuit applied at the terminals of either winding) that causes a significant increase in electrical current in the windings. Such a fault generates forces that attempt to radially inwardly collapse the radial inner winding and radially outwardly burst the radial outer winding. The magnitude of the collapsing and/or bursting force depends on the magnitude of the current generated by the fault, the number of turns in the winding and the ratio of the coil diameter to the coil height. In addition, axial misalignment between the windings will cause current due to the fault to generate axial forces that tend to apply a shear effect (i.e. substantially parallel to the longitudinal axis of the windings) on the interwinding supports. The magnitude of the axial, or shear, force depends on the degree of axial misalignment between the windings and the magnitude of the current generated by the fault.
Additionally, for efficient transformer operation, the support system must be able to accommodate thermal forces from coil temperatures above ambient expected during operation due to I.sup.2 R losses in the conductors, from eddy currents and hysteresis losses in the core, and from stray flux impinging the axial ends of the windings. Further, the support system must restrain vibratory forces during operation. In certain transformers, the elements of the core of the transformer are fabricated from a plurality of laminations. As is well known, laminations of the core of a transformer are subject to vibratory forces between the laminations during operation due to electrical currents, such as eddy currents, induced in the laminations by the magnetic flux of the transformer, which result in magnetostrictive forces within the core. The support system must satisfactorily restrain, resist and withstand all these forces over long term operation, yet be easily and readily fabricated in order to minimize cost.
One transformer of the type addressed by the present invention is a liquid cooled transformer which may be used, for example, in the excitation system of a large dynamoelectric machine. Such an excitation system and associated transformer is described in a copending application entitled, "Liquid Cooled Static Excitation System For A Dynamoelectric Machine", having Ser. No. 776,331, filed on Sept. 16, 1985, and assigned to the present assignee. It is expected that the present invention may be most beneficially applied to transformers having a rating from about 3000 KVA to about 10,000 KVA. However, expression of ratings is not intended to limit application of the invention. In a generator of this category, a typical operating temperature rise of about 15.degree. C. over a water coolant input temperature of about 45.degree. C. may be experienced.
Accordingly, it is an object of the present invention to provide a coil support system for a transformer wherein the coil support system provides both radial and axial support to the coils forming primary and secondary windings of the transformer adequate to restrain and withstand forces due to a fault, such as a sudden short circuit applied at the terminals of a winding.
Another object of the present invention is to provide a coil support system that is able to accommodate thermal forces from coil temperatures above ambient expected during operation.
Yet another object of the present invention is to provide a coil support system that restrains and withstands vibratory forces on the laminations of the core of the transformer during operation.